To render Earth invisible to extraterrestrial eyes, powerful lasers may do the trick, say astronomers from Columbia University in New York.
In a theoretical paper published in the Monthly Notices of the Royal Astronomical Society, David Kipping and Alex Teachey outline how we could don a planetary cloak made of light, should aliens be looking in our direction.
It relies on an exoplanet-hunting technique called the "transit method". When a planet passes between us and its star, the star's light dips slightly, and we can detect this light difference.
While this only lets us "see" exoplanets directly on our plane, some three quarters of exoplanets discovered between 2005 and 2015 were found using the transit method, of which more than 80% were picked up by the Kepler space telescope.
And looking for a dip in starlight is just the tip of the iceberg. To zoom into habited (or potentially life-containing) exoplanets, "transit spectroscopy" looks for molecular signatures of life, such as oxygen or polluants from industrial activity, that absorb various wavelengths of starlight as it filters through.
We can evade detection as we orbit the Sun if we emit the amount of the light we block.
Because humans have nutted out the transit method, Kipping and Teachey write, it's feasible that other sufficiently advanced civilisations have too. And if they don't come in peace, it's in our best interests to fly under their radar (so to speak).
We can evade detection as we orbit the Sun if we emit the amount of the light we block, effectively cancelling the "dip" in starlight. One way might be to build huge controllable mirrors, but this would be very expensive.
So what about lasers?
Well, we already have laser technology. Some telescopes, such as the Very Large Telescope in Chile, use bright lasers to take clearer images.
Besides, we wouldn't even need an alien-tricking laser switched on all the time. The pair calculated we'd need just one 30-megawatt laser pointing at a star system that might be looking our way while we're in the detectable transit stage for 10 hours or so each year.
This "broadband cloak" could protect us in visible light, but leave us open to detection on either side of the light spectrum, such as in ultraviolet or infrared.
Extending the laser to those longer and shorter wavelengths would need an army of tunable lasers which can sweep across the spectrum at high speed. This "chromatic cloak" would be a power-guzzler: the pair calculated it to the tune of 250 megawatts.
And on the other end of the laser-power spectrum is the "biocloak", where only the atmospheric signatures that give away our presence – such as oxygen – are hidden.
This, Teachey says, "is achievable with a peak laser power of just 160 kilowatts per transit.
"To another civilisation, this should make the Earth appear as if life never took hold on our world."
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